The present invention is directed to the field of cryogenic cooling and is more specifically directed to apparatus and a method for cryogenically cooling materials in which temperatures can be decreased or increased continuously or in incremental steps while maintaining very low temperature gradients throughout the chamber in which the materials are treated.
Materials exposed to cryogenic temperatures show improvement in wear resistance. Commercial systems for cryogenic processing generally employ one of two methods: either exposing materials to vapors emanating from cold liquified gases, or cryogens, at or near standard pressure conditions for a predetermined time, or exposing materials to liquid-gas vapors and slowly lowering the items into the liquid gas itself at or near standard pressure conditions. Both types of systems have proven very successful, but have disadvantages. For example, when treating a material in vapors above a cryogenic liquid, a large thermal gradient between the vapor and liquid cryogen can exist. When a material is pre-cooled in a vapor and then lowered into the cryogenic liquid, an added disadvantage exists, in that the lower portion of the material enters the liquid-gas phase while the upper portion is still in the vapor phase. This creates some distortion within the material because of the temperature gradient between the upper-level vapor and the vapor-liquid interface.
Most commercial systems for cryogenic processing generally use liquid nitrogen for vapor treatment and liquid-gas immersion. Thus, the minimum temperature the material can be exposed to at standard pressure conditions is about -320.degree. F., the boiling point of nitrogen at standard pressure. If some other cryogen is used, the minimum temperature attained will also be that of its boiling point at standard pressure conditions. The ability of most commercial systems to control temperatures within thermal-fracture or distortion threshholds is also questionable. When treating a material in a cryogenic vapor or liquid at standard pressures or higher, the cooling capability of the cryogen is limited by the following factors:
(1) for an open system, the maximum heat absorption of the vapor cryogen is limited by the maximum expansion temperature of the vapor, which will be determined by the mass and temperature of its external surroundings and the boiling point of the liquid cryogen at standard pressure; PA1 (2) for a closed system, maximum heat absorption of the cryogen will be limited by the maximum temperature obtained, which in turn is limited by the higher pressure within the closed system. The minimum temperature which can be obtained within such a closed system is that of the liquid cryogen's boiling point, which will be a higher temperature than the boiling point at standard pressure.
It is the solution of these problems to which the present invention is directed.